The present invention relates to a system for monitoring a plurality of containers using ultrasonic seals. It is used in the monitoring of a large number of containers containing dangerous products, precious materials, documents, etc.
The principle of monitoring by ultrasonic seals is known. It consists of definitively fixing an ultrasonic seal to the container to be monitored, said seal being provided with a unique marking, which is not reproducible located in the vicinity of the predetermined seal breaking area. This marking is obtained by structural irregularities of the matrix containing the seal, e.g. by material inclusions or cavities distributed in a random manner.
This serves the double function of checking the identity of the marking and checking the integrity of the seal. It involves a comparison between a test signal from the seal and a stored reference signal.
Generally an ultrasonic seal comprises a transducer able to convert an electrical excitation into an ultrasonic wave and vice versa. The structural irregularities present on the path of the transmitted ultrasonic wave give rise to ultrasonic echoes which are detected by the transducer, the latter supplying an electric signal, whose complex form is specific to the seal. There are electronic means for exciting the transducer, collecting the electric signal which it supplies and analyzing said signal.
In such a device, the ultrasonic transducer works both as a transmitter and as a receiver, in such a way that the seal only has a single transducer. However, British patent application No. 79 14 203 describes a marked seal, whose matrix contains two piezoelectric transducers. However, in such a seal, one of the transducers is used for detecting the identity of the marking and the other for checking the integrity of the seal. Thus, each of these transducers serves as a transmitter and as a receiver of the ultrasonic signals.
Such seals are unable to operate sufficiently accurately and rapidly to be used in large numbers.